The invention relates to an automotive electrical alternator, and particularly to an alternator having a high output alternator bobbin.
This invention is related to an electrical alternator, particularly adapted for use in motor vehicle applications including passenger cars and light trucks. These devices are typically mechanically driven using a drive belt wrapped on a pulley connected to the crankshaft of the vehicle""s internal combustion engine. The belt drives a pulley on the alternator which rotates an internal rotor assembly to generate alternating current (AC) electrical power. This alternating current electrical power is rectified to direct current (DC) and supplied to the motor vehicle""s electrical bus and storage battery.
While alternators have been in use in motor vehicles for many decades, today""s demands on motor vehicle design, cost, and performance have placed increasing emphasis on the design of more efficient alternators. Today""s motor vehicles feature a dramatic increase in the number of electrical on-board systems and accessories. Such electrical devices include interior and exterior lighting, climate control systems, increasingly sophisticated power train control systems, vehicle stability systems, traction control systems, and anti-lock brake systems. Vehicle audio and telematics systems place further demands on the vehicle""s electrical system. Still further challenges in terms of the output capacity of the motor vehicle""s electrical alternators will come with the widespread adoption of electrically assisted power steering and electric vehicle braking systems. Compounding these design challenges is the fact that the vehicle""s electrical system demands vary widely, irrespective of the engine operating speed which drives the alternator and changes through various driving conditions.
In addition to the challenges of providing high electrical output for the vehicle electrical alternator, further constraints include the desire to minimize the size of the alternator with respect to under hood packaging limitations, and its mass which relates to the vehicle""s fuel mileage.
In addition to the need of providing higher electrical output, designers of these devices further strive to provide high efficiency in the conversion of mechanical power delivered by the engine driven belt to electrical power output. Such efficiency translates directly into higher overall thermal efficiency of the motor vehicle and thus into fuel economy gains. And finally, as is the case with all components for mass-produced motor vehicles, cost remains a factor in the competitive offerings of such components to original equipment manufacturers.
The vast majority of all vehicles manufactured today use front-end accessory drive alternators that contain rotors that provide the alternator""s magnetic field and rotate within the machine. The magnetic field is generated when the field coil of the rotor, made up of a number of insulated copper wires wrapped around the steel pole piece hub, is energized and a current flows through the wire.
It is well known that the magnetic field strength that the rotor provides is proportional to the amount of power the alternator can provide to the vehicle""s systems. The field strength is increased by applying more voltage on the field coil resulting in more field current flowing through the windings. However, as the current increases in the field coil, the power dissipation in the form of heat increases at a rate that is squared due to the governing equation P=I2R. A well-known challenge in the art is to dissipate more heat from the hot copper field windings to the cooler steel pole pieces. It is critical that intimate contact between the field coil, insulating bobbin and the steel hubs of the poles be achieved with the thinnest possible separation between the coil and the hubs and the tightest contact.
Reducing the thermal contact resistance is very challenging since the rotor bobbin is wound with the field coil first. Then the field coil wound bobbin is assembled onto the hub of the pole pieces. To reduce the contact resistance between the field coil, insulating bobbin, and the steel hub the inside diameter of the bobbin is tightly fit over the outside diameter of the hub. However, the liberal tolerances of the various components create a variation in the fit of the bobbin over the hub, preventing a consistently tight fit of the components. Therefore, the rotor is designed for maximum power dissipation of the field coil as though the bobbin will always have a slip fit onto the hub.
Currently, one way of dealing with this problem is to ensure a very tight fit of the bobbin and coil onto the hub of the pole. The field coil is tightly wound onto the bobbin to create a coil-bobbin assembly. The coil-bobbin assembly is press fit onto the steel hub of the pole. This requires the inside diameter of the bobbin to stretch, which in turn stretches the field coil wire. The bobbin must be made of a flexible material. The problem with this design is that during the assembly process, the steel hub tends to rub on the inside diameter of the flexible bobbin causing it to tear and pinch between the steel hub pieces. This greatly limits the amount of press fit that can be obtained with this approach and prevents acquiring the desired fit between the components.
Another reason to obtain a tight press fit of the coil-bobbin assembly onto the pole hub is to help lock the coil-bobbin assembly to the hub and prevent slip between the bobbin and hub. During the operation of the alternator, the rotor accelerates and decelerates at very high rates as the engine speed changes. This results in a rotational force on the bobbin encouraging the bobbin to break free from the steel pole pieces. A very tight press fit locks the coil-bobbin assembly in place eliminating the need for additional locking features on the bobbin that take up space that could be used for field coil wire or the steel poles.
The present invention provides a method of obtaining a very tight press fit between the coil-bobbin assembly and the pole hub without the concern of pinching the bobbin material between the pole hub surfaces. The tighter fit means substantially lower thermal contact resistance between the field coil wire, bobbin and pole hub and, therefore, much higher field coil power dissipation capability. The end result is a higher power density alternator.
The present invention resolves the problems outlined above by providing a split or expandable seam in the bobbin. This allows the bobbin to be made of a more rigid material and still expand over the pole hub. A substantial press fit between the bobbin inner diameter and hub outer diameter can be obtained using this design. The bobbin expands and stretches the field coil wire resulting in securely locking the wire to the bobbin and the bobbin to the pole hub. The bobbin can be made from a more rigid material than could be used prior to the invention. For instance, steel can be used for the bobbin with a thin insulating layer wrapped around the outside diameter to insulate the wire from the steel hub. Use of a rigid material decreases pinching between the pole hubs during assembly. Additionally, the contact force between the field coil wire, bobbin, and pole hub can be increased substantially leading to a significant improvement in heat transfer from the field coil wire. Therefore, the field coil current can be increased without fear of overheating the coil resulting in a higher power density alternator.
Use of more rigid materials for the cylindrical diameter of the bobbin also allows the rotor to be filled with more field coil windings given the same allowable space in the rotor. When the coil-bobbin is assembled onto the poles, the inside finger angle of the poles that contacts the wire tends to force the wire in an inward direction toward the shaft. The resulting force on the wire tends to push the bobbin and field coil wire into the gap between the pole pieces during the assembly process. Use of a stronger, more rigid material for the cylindrical portion of the bobbin allows a significantly higher crushing force to be applied by the undersides of the fingers onto the field coil without the field coil wire or bobbin becoming displaced in a gap. Therefore, more wire can be used to create the field coil and allow the crushing force to increase substantially without concern of pinching material between the pole pieces.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.